The present invention relates to the field of semiconductor assembly and packaging, and in particular to a wire spool system for a wire bonding apparatus.
Wire bonders are used during semiconductor assembly and packaging for making electrical interconnections between electrical contact pads on a semiconductor die and a substrate, or between electrical contact pads on different semiconductor dies. Wire is fed from a wire spool containing bonding wire to a bonding tool such as a capillary for performing wire bonding.
A typical method used to bond or weld the wire to a connection pad is through a combination of heat, pressure and/or ultrasonic energy. It is a solid phase welding process, wherein the two metallic materials (the wire and the pad surface) are brought into intimate contact. Once the surfaces are in intimate contact, electron sharing or inter-diffusion of atoms takes place, resulting in the formation of a wire bond. The two main types of wire bond are ball bonds and wedge bonds.
For example, a wedge bonder bonds wires using ultrasonic energy generated by an ultrasonic transducer. The transducer produces ultrasonic oscillations. The oscillations pass through a wedge at a bond head of the wedge bonder, and are transmitted to the bonding wire underneath the wedge.
The bonding wire is fed out from the wire reel by the wire spooling system. A known wire spool is shown in FIG. 1. The wire spool usually comprises the following features: a spool shaft 101 which holds the wire reel, a tensioner 102 to preload the wire feeding out from the reel, a wire buffer region 103 to maintain a consistent feed of the bonding wire, a wire path 104 for feeding the bonding wire to the bond head, and a wire loss detector 105 which detects the wire movement after bonding in order to determine whether or not the bond has stuck.
The sticking of the bond can be determined by the wire loss detector 105 through detecting the movement of the bonding wire. Normally after a successful bond, the bond head will move up and the bonding wire will pay out from the bond tip for further processing such as looping and cutting. The amount of the wire pay out can be determined at the exit of the wire spool. If the amount is larger than a predetermined threshold, the bond can be treated as a stuck bond.
There are two types of wire loss detectors: contact and non-contact type. An example of a contact type detection mechanism is shown in FIG. 2. The contact type detection mechanism comprises a rotary encoder 201 operatively coupled to a pair of clamping rollers 202. The clamping rollers grip the bonding wire 203 in between to provide a frictional contact. When the wire 203 is fed, the rollers 202 will be rotated and hence the rotary encoder 201 will be able to record a signal representative of the length of wire which has been paid out.
An example of a non-contact wire loss detector is shown in FIG. 3. The non-contact wire loss detector comprises a sensor module having a line of photosensors 301. The sensor module is usually placed in the wire buffering zone (i.e., buffer zone 103 in FIG. 1). When the bonding wire 302 is fed, the looping of the wire in the buffer zone will displace (in direction 303) and trigger the photosensors 301. Detecting which sensor is triggered in the line will allow determination of the wire position and hence detection of the wire movement.
Each of the above arrangements has certain disadvantages. For a contact type wire loss detector, the detection sensitivity is quite low, because it is limited by the resolution of the encoder. Also, the inertia of the rollers will provide extra friction to the bonding wire, and will affect the consistency of looping of the wire as it is paid out during a bonding process. Moreover, due to the contact forces between the wire and the roller, the roller will tend to wear, hence causing detection inconsistency and wire contamination.
The non-contact type wire loss detector addresses most of the problems found in a contact type detector because it eliminates the contact interface between the wire and the roller. However, the detection sensitivity is still limited, because it is constrained by the sensor pitch 304 of the photosensor array 301. Accordingly, it cannot detect small wire movements.
Due to the low sensitivity of prior art detectors, sometimes wire loss detection for bonds with small wire feeds cannot be performed. For example, in a small diameter wire bonding application, the wire feeding before cutting is about the length of the bonding tool tip—of the order of a hundred microns—which usually cannot be detected.
There remains a need for a wire spool system which overcomes or alleviates at least one of the foregoing difficulties, or which at least provides a useful alternative.